The send() function initiates transmission of a message from the specified socket to its peer. The send() function sends a message only when the socket is connected (including when the peer of a connectionless socket has been set via connect()).
The length of the message to be sent is specified by the length argument. If the message is too long to pass through the underlying protocol, send() fails and no data is transmitted.
Successful completion of a call to send() does not guarantee delivery of the message. A return value of -1 indicates only locally-detected errors.
If space is not available at the sending socket to hold the message to be transmitted and the socket file descriptor does not have O_NONBLOCK set, send() blocks until space is available. If space is not available at the sending socket to hold the message to be transmitted and the socket file descriptor does have O_NONBLOCK set, send() will fail. The select() and poll() functions can be used to determine when it is possible to send more data.
The socket in use may require the process to have appropriate privileges to use the send() function.
#include ssize_t send(int socket, const void *buffer, size_t length, int flags);
Upon successful completion, send() returns the number of bytes sent. Otherwise, -1 is returned and errno is set to indicate the error.